Series-connected thermionic energy converters

ABSTRACT

A series-connected assembly of thermionic devices for the generation of electrical power having a series of cylindrically spaced apart, axially aligned, heated cathodes, a tubular cooled anode surrounding each cathode, interelectrode leads connecting each anode to its adjacent cathode, an ionization promoting vapor filling the interelectrode gaps characterized by shielding members positioned between the cathodes and the interelectrode leads and their adjacent higher potential cathodes, a tortuous path communicating between the interelectrode gaps of succeeding devices for circulating the ionization promoting vapor and an insulating member axially engaging the end of each cathode for supporting and centering the cathodes within the anodes.

United States Patent Jules et al.

[ Jan. 28, 1975 Primary Examiner-T. H. Tubbesing As is a t E am n r-1J1M- s s Attorney, Agent, or Firm Fleit, Gipple & Jacobson 57 ABSTRACT Aseries-connected assembly of thermionic devices for the generation ofelectrical power having a series of cylindrically spaced apart, axiallyaligned, heated cathodes. a tubular cooled anode surrounding eachcathode, interelectrode leads connecting each anode to its adjacentcathode, an ionization promoting vapor filling the interelectrode gapscharacterized by shielding members positioned between the cathodes andthe interelectrode leads and their adjacent higher potential cathodes, atortuous path communicating between the interelectrode gaps ofsucceeding devices for circulating the ionization promoting vapor and aninsulating member axially engaging the end of each cathode forsupporting and centering the cathodes within the anodes.

16 Claims, 3 Drawing Figures PATENTEU JAN 2 8 i975 SHEET 10F 2 INVENTORSEDWIN JULES WILLIAM J. LEVEDAHL BY W PATENTED JAN 2 8 I975 SHEET 2 BF 2INVENTORS EDWIN JULES WILLIAM J. LEVEDAHL SERIES-CONNECTED THERMIONICENERGY CONVERTERS This invention relates to thermionic energy convertersand more particularly to improved constructions for such devices forpromoting uniformity in the output power obtainable from such devicesand for facilitating the interconnection of a plurality of thermionicdevices in series.

Among the important considerations which affect the electrical poweroutput obtainable from thermionic devices such as diodes are those ofinterelectrode spacing in a single diode and interelectrode connectionsbetween successive series-connected diodes. The phenomena of electricalpower generation by the thermionic mechanism are essentiallythermodynamic. Thus, in the determination of the structure by whichinterelectrode separations are maintained and interelectrode connectionsare made, thermodynamic factors are exceedingly influential. Indeed, thethermal gradients in a thermionic power generation system aredeterminative of the electrical gradients. The electrical gradients onthe other hand, if not properly controlled, can alter the essentialthermal gradients and adversely effect the performance of the system.

The interelectrode spacing between anode and cathode in a thermionicdiode is critical. If it is not uniform the power density in the diodeis altered and the power output of the diode either declines or buildsto destructive peaks in particular areas of the diode. For example, ifone portion of the emitter surface on the cathode moves closer to thecollector surface of the anode in such a device during operation, eitherthrough structural failure or through lack of precision in theconstruction of the device, the increased local thermal gradient acrossthe interelectrode gap at that portion leads to a localization andconcentration of current flow. As a result, one portion of the cathodeand of the anode may become overloaded to the detriment of thethermodynamic properties of the device and other portions of the anodeand cathode then fail to function at peak efficiency.

In the generation of higher potentials by thermionic diodes it is commonto connect a plurality of diodes in series. Any two suchseries-connected thermionic diodes are capable of developing asignificant potential difference between them. Electrons emitted fromthe lower potential emitter or from the interelectrode lead connected toit can arc across to the emitter of the next succeeding diode resultingin a short circuit which greatly reduces the useful power output. Thisproblem can become far more serious when many diodes are connected inseries, for it is possible to develop an are along the entireseries-connected path from one end to the other through the cathodes andthereby effectively to short-circuit the entire chain of diodes.

This invention has a principal object to maximize the electrical powerobtainable from thermionic energy converters through the provision ofconstructions which promote uniformity of power output within and amonga number of devices in a system.

An additional object of this invention is to provide new and improvedthermionic diode constructions which properly balance the thermodynamicand electrodynamic principles upon which the operation of such devicesare based to improve the power distribution and power density andthereby to increase the operating efficiency of such devices.

This invention has as a still further object to provide means tomaximize the electrical power obtainable by thermionic diodes bypreventing arc-over between the electrodes of series-connected diodes.

It is a still further object of the invention to provide thermionicdiodes with improved means for maintaining the interelectrode spacingtherein both constant and uniform to improve the efficiency and poweroutput obtainable therefrom.

By way of a brief summary, the present invention may be embodied inthermionic diode constructions which act simultaneously to center acathode within a surrounding anode and to prevent arc-over betweenseries-connected diodes while maintaining consistant thermal gradientsbetween the cathodes and the anodes. In one form the invention employs ametallic back-emission shield, a relatively cold member interposedbetween two csthodes. The back-emission shield may comprise a disc-likemember connected to one anode and interposed between two cathodes atdifferent potentials. This back-emission shield is sufficiently thick toconduct any heat which is either received by it from radiation orgenerated within it by small electrical currents outward to therelatively cool collector or anode with little temperature gradient. Thebackemission shield is at essentially the same temperature andelectrical potential as is the anode. An interelectrode lead attached tothe collector at the same point as the back-emission shield and thus atthe same electrical potential with it is connected to the cathode of thenext succeeding thermionic diode. The interelectrode lead may have amuch smaller cross-sectional area than that of the interelectrode spacerto inhibit the flow of thermal energy therethrough to the anode withwhich it is connected. Thus, a substantial thermal gradient developsacross the interelectrode lead. An insulating member mounted at thecentral portion of either the back-emission shield or the interelectrodelead has a configuration which mates with that of the cathode mountedwithin the first anode and centers it axially therein.

Any electrons emitted by the interelectrode lead or by the cathode towhich it is connected and travelling in the direction of the adjacentcathode of more positive potential are collected by the back-emissionshield and are returned via an internal low resistance conducting pathproducing a low-loss internal electrical loop. Only electrons emitted bythe back-emission shield itself can reach the adjacent more positiveemitter. However, since the back-emission shield is at approximately thesame temperature as the anode to which it is connected, comparativelyfew electrons are emitted from it and the short circuiting effect isnegligible.

To provide proper out-gassing between the seriesconnected diodes andeffective pressure control of the ionization vapor, flow passages areprovided through the back-emission shield and the interelectrode lead.These passages may be located eccentrically such that the ionizationvapor in passing through them follows a tortuous route. Alternatively,the passages may be located near the perimeter of the shield and lead inpositions not aligned between successive cathodes. This avoids theformation direct line-of-sight emission path between successivelyinterconnected emitters without restricting the flow of the ionizationvapor through the series-connected diodes.

Although the scope of this invention is defined in the claims appendedhereto, further details of the invention as well as additional objectsand advantages will be more readily understood in connection with thefollowing description taken together with the annexed drawings in which:

FIG. 1 illustrates a longitudinal cross-sectional view of an assemblyfor the generation of electrical power constructed in accordance withthis invention and comprising a plurality of series-connected thermionicdiodes;

FIG. 2 is a view similar to that of FIG. 1 showing an alternateembodiment of the invention; and

FIG. 3 is a similar view of a still further embodiment of the invention.

Turning attention now to FIG. 1, an assembly may be seen in which aplurality of thermionic diodes a, 10b, 10c, and 10d are connected inseries. Throughout the following description the same reference numbersare employed to identify corresponding elements in the separate diodeswith letter subscripts to designate the particular diodes with whichthese elements are associated. Each diode includes a tubular anode ll ofcircular cross-section having a collector surface 12 on the inner sidethereof. Within each of the diodes is located a cylindrical cathode 13which may have a special emitter coating 14 on the external sidethereof. in the embodiment shown the cathode is heated by selfcontainedfissionable material bearing fuel solid 15.

The nature of the fuel solid 15 as well as its structure and containmentwithin the cathode 13 are not the subject of this invention. Preferablythese portions of the diode are constructed as described more fully andclaimed in a copending application Ser. No. 384,080 filed in the name ofJames Monroe on July 21. 1964.

The process by which large amounts of energy are released throughnuclear fission reactions is now quite well known. in general afissionable atom, in this case within the fuel solid 15, absorbs aneutron and its nucleus and undergoes a nuclear disintegration. Thisproduces fission products of lower atomic weight having great kineticenergy, and two or more neutrons also having high energy. The kineticenergy of the fission products is quickly dissipated in the fuel asheat. If, after this heat generation. there is at least one net neutronremaining which induces a subsequent fission, the fission reactionbecomes self sustaining and the heat generation by the fuel solid 15 iscontinuous. The heat generated internally raises the surface temperatureof the cathode 13 to a level above 1,500K, at which temperaturesubstantial numbers of electrons are emitted by the cathode 13 or itsemitter coating 14. These electrons travel across the interelectrodespace 16 to the collector surface 12 of the anode 11 therebytransporting a current and producing a substantial potential differencebetween the cathode l3 and the anode 11. To assist in the thermionicmechanism it is preferable for the interelectrode gap 16 to be filledwith the vapor of an alkali metal, such as cesium, at a pressure ofapproximately 0.5 millimeters of mercury.

Despite the narrowness of the interelectrode gap, which may be on theorder of 0.01 inch to 0.03 inch, for example, the anode ll should remainat a much lower temperature than that of the cathode 13 for bestresults. Therefore, cooling means are provided which are represented inthis case by a cooling jacket 20 surrounding the anodes 11. A coolant iscirculated through the jacket 20 to maintain the anodes at a temperaturebetween 600K and l,300K.

it is necessary that the anodes ll be electrically isolated from eachother. For this purpose annular insulators 23 are positioned betweenadjacent anodes l1 and may be sealed to one or both. A layer 24 ofelectrical insulation, preferably formed of a sleeve of M 0 alsosurrounds all of the anodes ll. Insulating layer 24 is. however,preferably quite thin. on the order of 0.01 inch, to promote the rapidtransfer of heat therethrough. A metal sheath 25 surrounds theinsulating layer 24 and is in direct contact with the coolant in jacket20. The insulating layer 24 may be bonded either to the anode 11 or tothe metal sheath 25 or to both.

in the practice of this invention each anode 11 is provided at one ofits ends with an integral back-emission shield 30 which is formed of ahigh conductivity material, preferably the same material employed forthe anode itself, and is shaped into a somewhat disc-like configuration.Shield 30 is of substantial cross-section so that despite its proximityto cathode 13, it is at approximately the same temperature as anode 11with a comparatively small internal thermal gradient.

in this embodiment the back-emission shield 30 has a central aperture 31within which is positioned a flanged support member 32. Support member32 is preferably formed of a material such as sintered M 0 which is agood electrical insulator. Support member 32 includes a central neck 33of reduced crosssectional diameter which fits within the aperture 31 inthe back-emission shield. Within neck 33 an axially centered bore 34 ofstill smaller diameter is provided.

It is to be noted that cathodes l3 carry on their lower domed end caps35 a circular extension 36 which fits closely within the bore 34 of theinsulating support member 32. Either bore 34 or extension 36 or both maybe tapered slightly to effect a precise axial alignment between themembers thereby determining the concentricity of one end of each cathode13 within its surrounding coaxial anode ll and fixing the interelectrodegap 16. Although the entire surface of each of the cathodes 13 includingthe end caps 35 is at a temperature elevated substantially above that ofthe associated anodes, the combination of a thermal insulating supportmember 32 within a shield 30 of high conductivity and broadcross-sectional area results in the interposition of a relatively coolmember between cathodes 13a and 13b and between cathodes 13b and and soon down the series.

Also in accordance with this invention an interelectrode lead 40 isprovided to connect the anode ll of each diode to the cathode 13 of thenext succeeding diode. Interelectrode lead 40, which is somewhat conicalin shape, is affixed at its outer diameter to the anode 11 at the pointof juncture 41 between the anode and the back-emission shield 30. At itsinnermost diameter the interelectrode lead 40 terminates in contact witha circular flange 42 on the cathode 13 in the next succeeding diode,establishing an equipotentiality between the anode of one diode and thecathode of the next.

The cross-sectional current-carrying thickness of interelectrode lead 40is, however, thinner than that of the back-emission shield 30, andpreferably is as thin as possible consistent with the magnitude of thecurrents which it must conduct. Thus, a substantial thermal gradientdevelops across the interelectrode lead 40 between its hot innermost endat 42 and its cooler outermost end at 4]. ln this embodiment theinterelectrode lead 40 also functions as a structural centering memberto locate the uppermost end of each of the cathodes as seen in thisillustration and thereby to determine with precision the thickness ofthe interelectrode gap 16.

In filling the interelectrode gaps of the respective diodes with vaporsof cesium or other alkaline metals it is desirable to have somecontinuity between the several diodes so that the pressure of theionizing vapor may be controlled in all diodes simultaneously and withthe same structure. The back-emission shield 30 is therefore providedwith at least one port 43 through which the cesium vapor may flow andthe interelectrode lead 40 is also provided with a similar port 44.These ports are positioned eccentrically with respect to one another toprovide a tortuous path for the flow of the cesium vapor thereby tominimize the possibility of the formation of line-of-sight arcs throughthe cesium vapor from cathode to cathode.

Attention is directed to the alternate form of the invention shown inFIG. 2. The form and arrangement of the components of this assembly aresimilar to those in the preceding embodiment and for this reasoncorresponding elements of the assembly are identified by the samereference numbers as are employed in connection with the firstembodiment. A major difference present in this second embodiment residesin the use of the back-emission shield 50 to determine the axialorientation of the cylindrical cathodes on both sides of it. Thus, thedisc-like emission shield 50 incorporates at the central portion thereofa recess 51 within which is positioned a cup-shaped support memberformed of an electrically and thermally insulating material. The axialextension 54 on the cathode 13 above fits tightly and precisely withinthe cavity 55 formed in the insulating support member 52. Theinterelectrode gap 16 and particularly its uniformity is determined inpart by the precision of the centering of the cathode extension 54within the central bore 55 in the insulating member 52.

The configuration of the back-emission shield 50 which provides therecess 51 on one side of the shield results in a dome-like knob 56 onthe opposite surface. Over this knob is fitted a platelike layer 57 ofan insulating material. This latter layer 57 fits within the flangedring 58 projecting symmetrically from the top end of the next succeedingcathode. It can thus be seen that the back-emission shield 50 with itsinsulators 52 and 57 determines the interelectrode spacing of thecathode structures on both sides of it.

In this embodiment the roughly conical interelectrode lead 60 is notemployed in a dual capacity to assist in the centering of the cathode.lnterelectrode lead 60 is again attached to the anode 11 at its point ofjuncture 61 with the back-emission shield 50 and extends into electricalcontact with the upper surface of the next succeeding cathode. Alsobecause of the reduced cross-sectional current-carrying thickness of theinterelectrode lead 60, a substantial thermal gradient is developedacross it although it maintains the anode 11 to which it is connected ata potential equal to that of the next succeeding cathode 13.

A tortuous path for the flow of ionization-promoting vapors betweendiodes is provided by one or more channels 65 in shield 50 and one ormore channels 66 in interelectrode lead 60. This tortuous path, as inthe previous example, results in a path between adjacent diodes which isnot on a line-of-sight between respective cathodes.

A still further embodiment is shown in FIG. 3 wherein, once again,similar reference numbers are employed to identify elements of thecombination which are substantially identical with correspondingelements in the preceding illustrations. in this embodiment therelatively cool back-emission shield 70 has only the one function ofpreventing short circuiting emission paths between cathodes and does notparticipate in the centering of the cathodes 13 within the anodes ll.

The centering function is fulfilled in part in this embodiment by theinterelectrode connector 7] which is separated from the back-emissionshield 70 by a narrow space 72. The interelectrode connector 71 extendsfrom the lower end 73 of the anode 11 to which it is connected intoengagement with the rolled-over lip 74 on the top surface of the cathode13 in the next succeeding thermionic device. This engagement not onlyconnects the upper anode lla electrically to the next succeeding cathode13b, but also centers the lower cathode 13b in place, determining theuniformity of the interelectrode gap 16b.

The upper cathode 13a is centered by means of an insulating supportmember 75 which. nested within a recess 76 in the upper end of the lowercathode 13b, receives within its bore 77 a centering projection 78 onthe lower end of the upper cathode l3a. thereby fixing the upper cathode13a on the axis of symmetry of the assembly. A path for the circulationof ionization vapors through the series-connected diodes avoidinglineof-sight routes between successive cathodes may be provided in thisinstance by nonaligned apertures 80 and 81 through the back-emissionshield 70 and the interelectrode connector 71 respectively.

It is to be noted that the back-emission shield 70 terminates at itsinnermost portion short of contact with any other member. Specifically,it does not support or contact the insulating support member 75. Thisarrangement results in the back-emission shield 70 having a somewhatsmaller thermal gradient and cooler average temperature than in theprevious two examples of the invention.

At this point it should be apparent that there have been shown anddescribed thermionic diode assemblies which satisfy the objectives ofthe invention and provide the advantages referred to above. While butthree examples have been illustrated herein, it is to be understood thatthese examples are offered as illustrative of the invention and notnecessarily to limit the scope thereof. Other variations andmodifications of the invention in addition to those specifically shownand described herein will doubtless occur to those skilled in the art towhich the invention pertains and the appended claims are intended toencompass all such variations and modifications as fall within the truespirit and scope of the invention in its broader aspects.

What is claimed is:

1. A series-connected assembly of thermionic devices for the generationof electrical power comprising:

a series of cylindrical spaced apart axially aligned heated cathodes;

a series of tubular axially aligned cooled anodes each surrounding arespective one of said cathodes;

a series of interelectrode leads electrically connecting said anodes tosaid cathodes of adjacent said devices; and

a series of conductive back emission shields, each joined to arespective one of said anodes at one end thereof and extending into thespaces between adjacent cathodes and between said interelectrode leadsand their adjacent higher potential said cathodes, said shieldsremaining substantially cooler than said cathodes by virtue of theirconductivity and their connections to said cooled anodes.

2. A series-connected assembly of thermionic devices for the generationof electrical power comprising:

a series of cylindrical, spaced-apart and axially aligned heatedcathodes;

a series of tubular axially aligned cooled anodes each surrounding arespective one of said cathodes;

at least one insulating support member for supporting and centering saidcathodes within said anodes with interelectrode gaps therebetween:

interelectrode connectors connected from the anode of each device to thecathode of the next succeeding device, said connectors developing asubstantial thermal gradient thereacross by virtue of the directconnections between a heated electrode at one end and a cooled electrodeat the other; and

a series of conductive metallic shields, each joined to a respective oneof said anodes at one end thereof and extending into the spaces betweenadjacent cathodes and between said interelectrode connectors and theiradjacent higher potential said cathodes, said shields remainingsubstantially cooler than said cathodes by virtue of their conductivityand their connections to said cooled anodes.

3. A series-connected assembly of thermionic devices for the generationof electrical power comprising:

a series of cylindrical, spaced-apart and axially aligned heatedcathodes;

a series of tubular axially aligned cooled anodes each surrounding arespective one of said cathodes;

a series of conductive metallic shields, each joined to a respective oneof said anodes at one end thereof and extending into the spaces betweenadjacent cathodes, said shields remaining substantially cooler than saidcathodes by virtue of their conductivity and their connections to saidcooled anodes;

at least one insulating support member mounted on a central portion ofeach of said shields for supporting and centering said cathodes withinsaid anodes with interelectrode gaps therebetween;

interelectrode connectors separate from said shields connected from theanode of each device to the cathode of the next succeeding device, saidconnectors developing a substantial thermal gradient thereacross byvirtue of the direct connections between a heated electrode at one endand a cooled electrode at the other.

4. The combination, with a plurality of substantially identicalthermionic energy conversion devices for the generation of electricalpower each including a tubular cooled anode, a cylindrical heatedcathode mounted within said anode, and an interelectrode gap betweensaid anode and cathode filled with an ionizationpromoting vapor inoperation, comprising:

at least one support member of insulating material centering thecathodes on the axis of symmetry of said anodes and electricallyinsulated therefrom;

an interelectrode metallic connector connected from the cooled anode ofeach device to the heated cathode of the next succeeding device; and

a disc-like conductive back emission shield connected to one end of eachof said anodes and inwardly extending therefrom so as to be positionedbetween said interelectrode metallic connectors and their adjacenthigher potential said cathodes, said shield remaining substantiallycooler than said cathode by virtue of its conductivity and itsconnection with said cooled anode.

5. In combination, a plurality of thermionic devices for the generationof electrical power, said devices being connected electrically in seriesand each comprising:

a tubular anode;

a cylindrical cathode concentrically positioned within said anode andseparated therefrom by an interelectrode gap;

insulating support means engaging and centering said cathode within saidanode;

means for heating said cathode to an elevated temperature above that ofsaid anode;

an interelectrode connecting member joined to said anode and extendinginwardly and axially into engagement with the cathode in the nextsucceeding thermionic device; and

an annular metallic shield connected at one end of said anode andextending inwardly therefrom between said cathode and saidinterelectrode connecting member.

6. A series-connected assembly of thermionic diodes for the generationof electrical power comprising:

a series of cylindrical spaced-apart and axially aligned heated athodes;

a series of tubular axially aligned cooled anodes each surrounding arespective one of said cathodes;

at least one insulating support member for supporting and centering saidcathodes within said anodes with interelectrode gaps therebetween;

an ionization-promoting vapor filling each of said interelectrode gapsin operation;

interelectrode connectors electrically connecting the anode of eachdiode to the cathode of the next succeeding diode, said connectorsdeveloping a substantial thermal gradient thereacross by virtue of thedirect connections between a heated electrode at one end and a cooledelectrode at the other; and

a series of conductive metallic shields, each joined to a respective oneof said anodes at one end thereof and extending into the spaces betweenadjacent cathodes and between said interelectrode connections and theiradjacent higher potential said cathodes, said shields remainingsubstantially cooler than said cathodes by virtue of their conductivityand their connections to said cooled anodes, there further being an openpathway between successive interelectrode gaps for the flow of saidionization promoting vapor therethrough.

7. The combination, with a plurality of substantially identicalthermionic energy conversion devices for the generation of electricalpower each including a tubular cooled anode, a cylindrical heatedcathode mounted within said anode, and an interelectrode gap betweensaid anode and cathode filled with an ionizationpromoting vapor inoperation, of means for connecting said devices electrically in serieswith substantially equipotential connections between the anode of onedevice and the cathode of the next succeeding device and for accuratelycentering the cathode within the anode of each of said devices toprovide a preselected interelectrode gap therebetween, said meanscomprising:

at least one axially mounted support member of insulating material, eachof said cathodes having at at least one end a configuration mating withthat of said support member to center the cathodes on the axis ofsymmetry of said anodes and electrically insulated therefrom;

a disc-like conductive back emission shield of metal connected to oneend of each of said anodes and inwardly extending therefrom, said shieldremaining substantially cooler than said cathode by virtue of itsconductivity and its connection with said cooled anode; andaninterelectrode metallic connector separate from said shield connectedfrom the cooled anode of each energy conversion device to the heatedcathode of the next succeeding device, there being a tortuous pathcommunicating between the interelectrode gaps of succeeding devices forthe circulation therethrough of the aforesaid ionizationpromoting vapor.

8. In combination, a plurality of thermionic devices for the generationof electrical power, said devices being connected in series and eachcomprising:

a tubular anode;

a tubular cathode having an electron-emissive surface concentricallypositioned within said anode and separated therefrom by aninterelectrode gap;

insulating support means engaging and centering said cathode within saidanode;

means for heating said cathode to an elevated temperature above that ofsaid anode;

an ionization-promoting vapor filling said interelectrode gap inoperation;

an interelectrode connecting member joined to said anode and extendinginwardly and axially into engagement with the cathode in the nextsucceeding thermionic device;

an annular metallic shield connected at one end of said anode andextending inwardly therefrom so as to be positioned between saidinterelectrode connecting member and its adjacent higher potential saidcathode; and

means defining pathway between the interelectrode gaps of successivedevices for the circulation therethrough of said ionization-promotingvapor.

9. In combination: a plurality of axially aligned thermionic diodes forthe generation of electrical power, said diodes being connected inseries and each comprising:

a tubular cooled anode;

a tubular heated cathode having an electron-emissive surfaceconcentrically positioned within said anode and separated therefrom byan interelectrode gap;

at least one insulating support member engaging and centering saidcathode within said anode;

a metallic shield at one end of said anode extending inwardly therefromand cooled by contact with said anode;

an interelectrode connecting member joined to said cooled anodeextending inwardly and axially into engagement with the heated cathodein the next succeeding thermionic diode; and

means defining a torluous pathway between the interelectrode gaps inadjacent diodes for control of the composition and pressure of vapors insaid interelectrode gaps.

10. A series-connected assembly of thermionic devices for the generationof electrical power comprising:

a series of cylindrical, spaced-apart and axially aligned heatedcathodes;

a series of tubular axially aligned cooled anodes each surrounding arespective one of said cathodes;

at least one insulating support member supporting and centering saidcathodes within said anodes with interelectrode gaps therebetwwen;

a series of conductive metallic shields, each joined to a respective oneof said anodes at one end thereof and extending into the spaces betweenadjacent cathodes, said shields remaining substantially cooler than saidcathodes by virtue of their conductivity and their connections to saidcooled anodes;

an ionization-promoting vapor filling each of said interelectrode gapsin operation; and

annular interelectrode connectors separate from said shields, eachconnected at its periphery to the anode of one of said devices andextending into contact with the cathode of the next succeeding device,said connectors developing a substantial thermal gradient thereacross byvirtue of the direct connections between a heated electrode at one endand a cooled electrode at the other, there being an open'pathway betweensuccessive interelectrode gaps following a tortuous route through eachof the structures comprising an anode, a shield and an interelectrodeconnector for the flow of ionizationpromoting vapors therethrough.

11. The combination, with a plurality of substantially identical axiallyaligned thermionic energy conversion diodes for the generation ofelectrical power each including a tubular cooled anode, and acylindrical heated cathode mounted within said anode, with aninterelectrode gap between said anode and cathode filled with anionization-promoting vapor in operation, of means for connecting saiddiodes electrically in series with substantially equipotentialconnections between the anode of one diode and the cathode of the nextsucceeding diode and for accurately centering the cathode within theanode of each of said diodes to provide a predetermined interelectrodegap therebetween, said means comprising:

at least one axially mounted support member of insulating material, eachof said cathodes having at at least one end a configuration mating withthat of said support member to center the cathode on the axis ofsymmetry of said anode and electrically insulated therefrom;

a disc-like conductive back emission shield of metal connected to oneend of each of said anodes and inwardly extending therefrom, said shieldremaining substantially cooler than said cathode by virtue of itsconductivity and its connection with said cooled anode; and

annular interelectrode metallic connectors separate from said shields,each connected at its periphery to the cooled anode of one diode andextending into contact with the heated cathode of the next succeedingdiode, there being a tortuous path communicating between theinterelectrode gaps of succeeding diodes for the circulationtherethrough of the aforesaid ionization-promoting vapor.

l2. ln combination: a plurality of axially aligned thermionic diodes forthe generation of electrical power, said diodes being connectedelectrically in series and each comprising:

a tubular anode;

a cylindrical cathode having an emitter surface thereon concentricallypositioned within said anode and separated therefrom by aninterelectrode gap;

insulating support means engaging and centering said cathode within saidanode;

means for heating said cathode to an elevated temperature above that ofsaid anode;

an ionization-promoting vapor filling said interelectrode gap inoperation;

an annular metallic shield connected at one end of said anode andextending inwardly therefrom;

an interelectrode connecting member joined to said anode adjacent theregion ofjuncture between said anode and said shield and extendinginwardly and axially into engagement with the cathode in the nextsucceeding thermionic diode; and

means defining a tortuous path through the structure between adjacentdiodes in communication with the interelectrode gaps thereof for thecirculation therethrough of said ionization-promoting vapor.

13. ln combination: a plurality of axially aligned thermionic diodes forthe generation of electrical power, said diodes being connectedelectrically in series and each comprising:

a tubular anode;

a cylindrical cathode having an emitter surface thereon concentricallypositioned within said anode and separated therefrom by aninterelectrode gap;

means for heating said cathode to an elevated temperature above that ofsaid anode;

an ionization-promoting vapor filling said interelectrode gap inoperation;

an annular metallic shield connected at one end of said anode andextending inwardly therefrom;

insulating support means borne by said shield at a central portionthereof, said insulating support means engaging and centering saidcathode within said anode;

an interelectrode connecting member joined to said anode adjacent theregion of juncture between said anode and said shield and extendinginwardly and axially into engagement with the cathode in the nextsucceeding thermionic diode; and means defining a tortuous path throughthe structure between adjacent diodes in communication with theinterelectrode gaps thereof for the circulation therethrough of saidionization-promoting vapor. 14. A series-connected assembly ofthermionic devices for the generation of electrical power comprising: aseries of cylindrical, spaced apart, axially aligned,

heated cathodes; a series of tubular, axially aligned, cooled anodes,each surrounding a respective one of said cathodes; a series ofinterelectrode leads electrically connecting said anodes to saidcathodes of adjacent said devices; and means for preventing electronsemitted from said interelectrode leads and said cathodes from arcing totheir adjacent higher potential said cathodes. 15. A series-connectedassembly of thermionic devices for the generation of electrical powercomprising: a series of cylindrical, spaced apart, axially aligned,

heated cathodes; a series of tubular, axially aligned, cooled anodes,each surrounding a respective one of said cathodes; a series ofinterelectrode leads electrically connecting said anodes to saidcathodes of adjacent said devices; and means positioned between saidinterelectrode leads and their adjacent higher potential said cathodesfor preventing electrons arcing from said interelectrode leads to saidadjacent higher potential said cathodes. 16. A series-connected assemblyof thermionic devices for the generation of electrical power comprising:a series of cylindrical, spaced apart, axially aligned,

heated cathodes; a series of tubular, axially aligned, cooled anodes,each surrounding a respective one of said cathodes; a series ofinterelectrode leads electrically connecting said anodes to saidcathodes of adjacent said devices; and means, including an insulatingsupport member, axially engaging the end of each said cathode forsupporting and centering said cathodes within said anodes withuninterrupted interelectrode gaps therebetween.

1. A series-connected assembly of thermionic devices for the generationof electrical power comprising: a series of cylindrical spaced apartaxially aligned heated cathodes; a series of tubular axially alignedcooled anodes each surrounding a respective one of said cathodes; aseries of interelectrode leads electrically connecting said anodes tosaid cathodes of adjacent said devices; and a series of conductive backemission shields, each joined to a respective one of said anodes at oneend thereof and extending into the spaces between adjacent cathodes andbetween said interelectrode leads and their adjacent higher potentialsaid cathodes, said shields remaining substantially cooler than saidcathodes by virtue of their conductivity and their connections to saidcooled anodes.
 2. A series-connected assembly of thermionic devices forthe generation of electrical power comprising: a series of cylindrical,spaced-apart and axially aligned heated cathodes; a series of tubularaxially aligned cooled anodes each surrounding a respective one of saidcathodes; at least one insulating support member for supporting andcentering said cathodes within said anodes with interelectrode gapstherebetween: interelectrode connectors connected from the anode of eachdevice to the cathode of the next succeeding device, said connectorsdeveloping a substantial thermal gradient thereacross by virtue of thedirect connections between a heated electrode at one end and a cooledelectrode at the other; and a series of conductive metallic shields,each joined to a respective one of said anodes at one end thereof andextending into the spaces between adjacent cathodes and between saidinterelectrode connectors and their adjacent higher potential saidcathodes, said shields remaining substantially cooler than said cathodesby virtue of their conductivity and their connections to said cooledanodes.
 3. A series-connected assembly of thermionic devices for thegeneration of electrical power comprising: a series of cylindrical,spaced-apart and axially aligned heated cathodes; a series of tubularaxially aligned cooled anodes each surrounding a respective one of saidcathodes; a series of conductive metallic shields, each joined to arespective one of said anodes at one end thEreof and extending into thespaces between adjacent cathodes, said shields remaining substantiallycooler than said cathodes by virtue of their conductivity and theirconnections to said cooled anodes; at least one insulating supportmember mounted on a central portion of each of said shields forsupporting and centering said cathodes within said anodes withinterelectrode gaps therebetween; interelectrode connectors separatefrom said shields connected from the anode of each device to the cathodeof the next succeeding device, said connectors developing a substantialthermal gradient thereacross by virtue of the direct connections betweena heated electrode at one end and a cooled electrode at the other. 4.The combination, with a plurality of substantially identical thermionicenergy conversion devices for the generation of electrical power eachincluding a tubular cooled anode, a cylindrical heated cathode mountedwithin said anode, and an interelectrode gap between said anode andcathode filled with an ionization-promoting vapor in operation,comprising: at least one support member of insulating material centeringthe cathodes on the axis of symmetry of said anodes and electricallyinsulated therefrom; an interelectrode metallic connector connected fromthe cooled anode of each device to the heated cathode of the nextsucceeding device; and a disc-like conductive back emission shieldconnected to one end of each of said anodes and inwardly extendingtherefrom so as to be positioned between said interelectrode metallicconnectors and their adjacent higher potential said cathodes, saidshield remaining substantially cooler than said cathode by virtue of itsconductivity and its connection with said cooled anode.
 5. Incombination, a plurality of thermionic devices for the generation ofelectrical power, said devices being connected electrically in seriesand each comprising: a tubular anode; a cylindrical cathodeconcentrically positioned within said anode and separated therefrom byan interelectrode gap; insulating support means engaging and centeringsaid cathode within said anode; means for heating said cathode to anelevated temperature above that of said anode; an interelectrodeconnecting member joined to said anode and extending inwardly andaxially into engagement with the cathode in the next succeedingthermionic device; and an annular metallic shield connected at one endof said anode and extending inwardly therefrom between said cathode andsaid interelectrode connecting member.
 6. A series-connected assembly ofthermionic diodes for the generation of electrical power comprising: aseries of cylindrical spaced-apart and axially aligned heated athodes; aseries of tubular axially aligned cooled anodes each surrounding arespective one of said cathodes; at least one insulating support memberfor supporting and centering said cathodes within said anodes withinterelectrode gaps therebetween; an ionization-promoting vapor fillingeach of said interelectrode gaps in operation; interelectrode connectorselectrically connecting the anode of each diode to the cathode of thenext succeeding diode, said connectors developing a substantial thermalgradient thereacross by virtue of the direct connections between aheated electrode at one end and a cooled electrode at the other; and aseries of conductive metallic shields, each joined to a respective oneof said anodes at one end thereof and extending into the spaces betweenadjacent cathodes and between said interelectrode connections and theiradjacent higher potential said cathodes, said shields remainingsubstantially cooler than said cathodes by virtue of their conductivityand their connections to said cooled anodes, there further being an openpathway between successive interelectrode gaps for the flow of saidionization promoting vapor therethrough.
 7. The combination, with aplurality of substantially identical tHermionic energy conversiondevices for the generation of electrical power each including a tubularcooled anode, a cylindrical heated cathode mounted within said anode,and an interelectrode gap between said anode and cathode filled with anionization-promoting vapor in operation, of means for connecting saiddevices electrically in series with substantially equipotentialconnections between the anode of one device and the cathode of the nextsucceeding device and for accurately centering the cathode within theanode of each of said devices to provide a preselected interelectrodegap therebetween, said means comprising: at least one axially mountedsupport member of insulating material, each of said cathodes having atat least one end a configuration mating with that of said support memberto center the cathodes on the axis of symmetry of said anodes andelectrically insulated therefrom; a disc-like conductive back emissionshield of metal connected to one end of each of said anodes and inwardlyextending therefrom, said shield remaining substantially cooler thansaid cathode by virtue of its conductivity and its connection with saidcooled anode; and an interelectrode metallic connector separate fromsaid shield connected from the cooled anode of each energy conversiondevice to the heated cathode of the next succeeding device, there beinga tortuous path communicating between the interelectrode gaps ofsucceeding devices for the circulation therethrough of the aforesaidionization-promoting vapor.
 8. In combination, a plurality of thermionicdevices for the generation of electrical power, said devices beingconnected in series and each comprising: a tubular anode; a tubularcathode having an electron-emissive surface concentrically positionedwithin said anode and separated therefrom by an interelectrode gap;insulating support means engaging and centering said cathode within saidanode; means for heating said cathode to an elevated temperature abovethat of said anode; an ionization-promoting vapor filling saidinterelectrode gap in operation; an interelectrode connecting memberjoined to said anode and extending inwardly and axially into engagementwith the cathode in the next succeeding thermionic device; an annularmetallic shield connected at one end of said anode and extendinginwardly therefrom so as to be positioned between said interelectrodeconnecting member and its adjacent higher potential said cathode; andmeans defining pathway between the interelectrode gaps of successivedevices for the circulation therethrough of said ionization-promotingvapor.
 9. In combination: a plurality of axially aligned thermionicdiodes for the generation of electrical power, said diodes beingconnected in series and each comprising: a tubular cooled anode; atubular heated cathode having an electron-emissive surfaceconcentrically positioned within said anode and separated therefrom byan interelectrode gap; at least one insulating support member engagingand centering said cathode within said anode; a metallic shield at oneend of said anode extending inwardly therefrom and cooled by contactwith said anode; an interelectrode connecting member joined to saidcooled anode extending inwardly and axially into engagement with theheated cathode in the next succeeding thermionic diode; and meansdefining a torluous pathway between the interelectrode gaps in adjacentdiodes for control of the composition and pressure of vapors in saidinterelectrode gaps.
 10. A series-connected assembly of thermionicdevices for the generation of electrical power comprising: a series ofcylindrical, spaced-apart and axially aligned heated cathodes; a seriesof tubular axially aligned cooled anodes each surrounding a respectiveone of said cathodes; at least one insulating support member supportingand centering said cathodes within said anodes with interelectrode gapstherebetwwen; a serIes of conductive metallic shields, each joined to arespective one of said anodes at one end thereof and extending into thespaces between adjacent cathodes, said shields remaining substantiallycooler than said cathodes by virtue of their conductivity and theirconnections to said cooled anodes; an ionization-promoting vapor fillingeach of said interelectrode gaps in operation; and annularinterelectrode connectors separate from said shields, each connected atits periphery to the anode of one of said devices and extending intocontact with the cathode of the next succeeding device, said connectorsdeveloping a substantial thermal gradient thereacross by virtue of thedirect connections between a heated electrode at one end and a cooledelectrode at the other, there being an open pathway between successiveinterelectrode gaps following a tortuous route through each of thestructures comprising an anode, a shield and an interelectrode connectorfor the flow of ionization-promoting vapors therethrough.
 11. Thecombination, with a plurality of substantially identical axially alignedthermionic energy conversion diodes for the generation of electricalpower each including a tubular cooled anode, and a cylindrical heatedcathode mounted within said anode, with an interelectrode gap betweensaid anode and cathode filled with an ionization-promoting vapor inoperation, of means for connecting said diodes electrically in serieswith substantially equipotential connections between the anode of onediode and the cathode of the next succeeding diode and for accuratelycentering the cathode within the anode of each of said diodes to providea predetermined interelectrode gap therebetween, said means comprising:at least one axially mounted support member of insulating material, eachof said cathodes having at at least one end a configuration mating withthat of said support member to center the cathode on the axis ofsymmetry of said anode and electrically insulated therefrom; a disc-likeconductive back emission shield of metal connected to one end of each ofsaid anodes and inwardly extending therefrom, said shield remainingsubstantially cooler than said cathode by virtue of its conductivity andits connection with said cooled anode; and annular interelectrodemetallic connectors separate from said shields, each connected at itsperiphery to the cooled anode of one diode and extending into contactwith the heated cathode of the next succeeding diode, there being atortuous path communicating between the interelectrode gaps ofsucceeding diodes for the circulation therethrough of the aforesaidionization-promoting vapor.
 12. In combination: a plurality of axiallyaligned thermionic diodes for the generation of electrical power, saiddiodes being connected electrically in series and each comprising: atubular anode; a cylindrical cathode having an emitter surface thereonconcentrically positioned within said anode and separated therefrom byan interelectrode gap; insulating support means engaging and centeringsaid cathode within said anode; means for heating said cathode to anelevated temperature above that of said anode; an ionization-promotingvapor filling said interelectrode gap in operation; an annular metallicshield connected at one end of said anode and extending inwardlytherefrom; an interelectrode connecting member joined to said anodeadjacent the region of juncture between said anode and said shield andextending inwardly and axially into engagement with the cathode in thenext succeeding thermionic diode; and means defining a tortuous paththrough the structure between adjacent diodes in communication with theinterelectrode gaps thereof for the circulation therethrough of saidionization-promoting vapor.
 13. In combination: a plurality of axiallyaligned thermionic diodes for the generation of electrical power, saiddiodes being connected electrically in series and each comprising: atubuLar anode; a cylindrical cathode having an emitter surface thereonconcentrically positioned within said anode and separated therefrom byan interelectrode gap; means for heating said cathode to an elevatedtemperature above that of said anode; an ionization-promoting vaporfilling said interelectrode gap in operation; an annular metallic shieldconnected at one end of said anode and extending inwardly therefrom;insulating support means borne by said shield at a central portionthereof, said insulating support means engaging and centering saidcathode within said anode; an interelectrode connecting member joined tosaid anode adjacent the region of juncture between said anode and saidshield and extending inwardly and axially into engagement with thecathode in the next succeeding thermionic diode; and means defining atortuous path through the structure between adjacent diodes incommunication with the interelectrode gaps thereof for the circulationtherethrough of said ionization-promoting vapor.
 14. A series-connectedassembly of thermionic devices for the generation of electrical powercomprising: a series of cylindrical, spaced apart, axially aligned,heated cathodes; a series of tubular, axially aligned, cooled anodes,each surrounding a respective one of said cathodes; a series ofinterelectrode leads electrically connecting said anodes to saidcathodes of adjacent said devices; and means for preventing electronsemitted from said interelectrode leads and said cathodes from arcing totheir adjacent higher potential said cathodes.
 15. A series-connectedassembly of thermionic devices for the generation of electrical powercomprising: a series of cylindrical, spaced apart, axially aligned,heated cathodes; a series of tubular, axially aligned, cooled anodes,each surrounding a respective one of said cathodes; a series ofinterelectrode leads electrically connecting said anodes to saidcathodes of adjacent said devices; and means positioned between saidinterelectrode leads and their adjacent higher potential said cathodesfor preventing electrons arcing from said interelectrode leads to saidadjacent higher potential said cathodes.
 16. A series-connected assemblyof thermionic devices for the generation of electrical power comprising:a series of cylindrical, spaced apart, axially aligned, heated cathodes;a series of tubular, axially aligned, cooled anodes, each surrounding arespective one of said cathodes; a series of interelectrode leadselectrically connecting said anodes to said cathodes of adjacent saiddevices; and means, including an insulating support member, axiallyengaging the end of each said cathode for supporting and centering saidcathodes within said anodes with uninterrupted interelectrode gapstherebetween.